Lakeshore Learning Materials 642 - Manual (Page 49)

Lake Shore Model 642 Electromagnet Power Supply User’s Manual
Operation 4-9
4.15 EXTERNAL CURRENT PROGRAMMING
The output current of the Model 642 can be set internally, externally, or by the sum of the external and internal settings.
Normally, the current is controlled internally by entering a setting from the front panel using the Output Setting key.
Refer to Paragraph 4.6 to set the output current. When the external program mode is set to External, the front panel
setting is fixed at 0 A and the output current is set using an external voltage where 10 V = 70 A. When the external
program mode is set to Sum, the internal and external settings are summed together to set the output current. When using
the External or Sum modes, care must be taken to insure that the output current does not exceed the maximum current for
the magnet. The software maximum setting limits cannot limit the output current or ramp rate that is set when using the
External or Sum modes. A –3 dB, 40 Hz, two-pole, low-pass filter limits the bandwidth of the external current
programming input. The bandwidth of the output is also limited by the compliance voltage and the inductance of the
magnet. To configure the external current program mode press External Program. The following setup screen appears
as a prompt for the external current program mode.
External Program Mode
Select With °®
External Program Mode: Internal
Use the s or t key to select the external current program mode, Internal, External, or Sum. Press Enter to accept the
new selection and return to the normal display. Press Escape to cancel the new selection and return to the normal
display.
To avoid discontinuities in the output current, the external current programming mode cannot be changed if the
programming voltage is not zero or the front panel current setting is not zero. If the external current program mode is
going to be kept from changing, an error box will pop up explaining why the new setting is being ignored. This error box
is shown below.
Change not allowed while
programming voltage is not zero.
4.16 LOCKING THE KEYPAD
The keypad lock feature prevents accidental changes to parameter values. When the keypad is locked, parameter values
may be viewed but cannot be changed from the front panel. The Model 642 has two keypad lock modes. The lock all
mode locks out changes to all parameters. The lock limits mode locks out changes to all of the parameters except Output
Setting, Ramp Rate, Zero Output and Pause Ramp. This allows the power supply to be operated without allowing any
changes to the power supply setup.
A 3-digit code must be used to lock and unlock the keypad. The factory default code is 123 and it can only be changed
using a computer interface. If the instrument parameters are reset to default values (See Default Parameter Values 4.18),
the code is reset to the factory default. The instrument parameters cannot be reset to default values from the front panel
when the keypad is locked.
The following message box appears on the display if the user attempts to change a parameter while the keypad is locked.
Change not allowed while
keypad is locked.
NOTE: The computer interface has a remote operation mode that may be mistaken for a locked keypad. If the front
panel Remote LED is lit, press the Local key to change to local control of the instrument.